Cardiac-arrest, exposure to high voltage power lines, and other trauma to the body can result in ventricular fibrillation. Ventricular fibrillation is the rapid and uncoordinated contraction of the myocardium of the heart. The use of external defibrillators to restore the heart beat to its normal pace through the application of an electrical shock is a well recognized and important tool in resuscitating patients. External defibrillation is typically used in emergency settings in which the patient is unconscious.
Automatic External Defibrillators (AEDs) are used by first responders such as police officers, fire fighters, and emergency medical technicians to resuscitate victims of sudden cardiac arrest. Studies have shown that the chances of successfully resuscitating a patient decrease approximately ten percent per minute following the onset of sudden cardiac arrest. Accordingly, a victim of sudden cardiac arrest will most likely not survive unless a trained rescuer responds in less than ten minutes after the cardiac arrest occurs and successfully defibrillates the heart.
Automatic External Defibrillators are designed to be very easy to use so that rescuers without extensive medical backgrounds can successfully resuscitate victims of sudden cardiac arrest. AEDs are currently being carried in emergency vehicles such as police cars, paramedic vehicles, and fire trucks. AEDs are also being widely deployed in areas where large numbers of people gather, such as at sports stadiums, gambling casinos, etc.
Because AEDs are portable devices for use in field locations, it is preferred that they be small and light weight. Because the AED is used in emergency situations far (e.g. more than ten minute travel time) from medical care facilities, the electo-therapy delivered by the AED must be effective at stopping ventricular fibrillation and returning the heart to a normal rhythm. It is desirable to lower electrical impedance across the patient and also to ensure an energy path that passes through the patient's heart. Current AEDs use two electrodes. See FIGS. 4 and 6. Two electrodes result in higher than desired patient impedance and make it difficult to establish an energy path that passes through the patient's heart.